Fix memory leak of SSL_CTX structures in TLS core.

Fix memory leak of SSL_CTX structures in TLS core.

SSL_CTX structures were allocated but never freed. This was a bigger
issue for clients than servers since new SSL_CTX structures could be
allocated for each connection. Servers, on the other hand, typically
set up a single SSL_CTX for their lifetime.

This is solved in two ways:

1. In __ssl_setup(), if a tcptls_cfg has an ssl_ctx on it, it is
freed so that a new one can take its place.
2. A companion to ast_ssl_setup() called ast_ssl_teardown() has
been added so that servers can properly free their SSL_CTXs.

(issue ASTERISK-19278)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@367002 65c4cc65-6c06-0410-ace0-fbb531ad65f3